1. Field
The present invention relates to broadcast communications, otherwise known as point-to-multipoint communications, in a wireline or a wireless communication system. More particularly, the present invention relates to a system and method for code combining at an outer decoder in such a broadcast communication system.
2. Background
Communication systems have been developed to allow transmission of information signals from an origination station to a physically distinct destination station. In transmitting the information signal from the origination station over a communication channel, the information signal is first converted into a form suitable for efficient transmission over the communication channel. Conversion, or modulation, of the information signal involves varying a parameter of a carrier wave in accordance with the information signal in such a way that the spectrum of the resulting modulated carrier is confined within the communication channel bandwidth. At the destination station the original information signal is replicated from the modulated carrier wave received over the communication channel. Such a replication is generally achieved by using an inverse of the modulation process employed by the origination station.
Modulation also facilitates multiple-access, i.e., simultaneous transmission and/or reception, of several signals over a common communication channel. Multiple-access communication systems often include a plurality of subscriber stations requiring intermittent service of relatively short duration rather than continuous access to the common communication channel. Several multiple-access techniques are known in the art, such as time division multiple-access (TDMA), frequency division multiple-access (FDMA), and amplitude modulation multiple-access (AM). Another type of a multiple-access technique is a code division multiple-access (CDMA) spread spectrum system that conforms to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wide-Band Spread Spectrum Cellular System,” hereinafter referred to as the IS-95 standard. The use of CDMA techniques in a multiple-access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE-ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” both assigned to the assignee of the present invention.
A multiple-access communication system may be a wireless or wire-line and may carry voice and/or data. An example of a communication system carrying both voice and data is a system in accordance with the IS-95 standard, which specifies transmitting voice and data over the communication channel. A method for transmitting data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION”, assigned to the assignee of the present invention. In accordance with the IS-95 standard, the data or voice is partitioned into code channel frames that are 20 milliseconds wide with data rates as high as 14.4 Kbps. Additional examples of communication systems carrying both voice and data comprise communication systems conforming to the “3rd Generation Partnership Project” (3GPP), embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), or “TR45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems, Release C” (the IS-2000 standard), also known as the 1xEV-DV proposal.
An example of a data only communication system is a high data rate (HDR) communication system that conforms to the TIA/EIA/IS-856 industry standard, hereinafter referred to as the IS-856 standard. This HDR system is based on a communication system disclosed in U.S. Pat. No. 6,574,211, entitled “METHOD and apparatus FOR HIGH RATE PACKET DATA transmission,” issued Jun. 3, 2003, and assigned to the assignee of the present invention. The HDR communication system defines a set of data rates, ranging from 38.4 kbps to 2.4 Mbps, at which an access point (AP) may send data to a subscriber station (access terminal, AT). Because the AP is analogous to a base station, the terminology with respect to cells and sectors is the same as with respect to voice systems.
In a multiple-access communication system, communications among users are conducted through one or more base stations. A first user on one subscriber station communicates to a second user on a second subscriber station by transmitting data on a reverse link to a base station. The base station receives the data and can route the data to another base station. The data is transmitted on a forward link of the same base station, or the other base station, to the second subscriber station. The forward link refers to transmission from a base station to a subscriber station and the reverse link refers to transmission from a subscriber station to a base station. Likewise, the communication can be conducted between a first user on one subscriber station and a second user on a landline station. A base station receives the data from the user on a reverse link, and routes the data through a public switched telephone network (PSTN) to the second user. In many communication systems, e.g., IS-95, W-CDMA, IS-2000, the forward link and the reverse link are allocated separate frequencies.
The above described wireless communication service is an example of a point-to-point communication service. In contrast, broadcast services provide point-to-multipoint communication service. The basic model of a broadcast system consists of a broadcast net of users served by one or more central stations, which transmit information with a certain contents, e.g., news, movies, sports events and the like to the users. Each broadcast net user's subscriber station monitors a common broadcast forward link signal. Because the central station fixedly determines the content, the users are generally not communicating back. Examples of common usage of broadcast services communication systems are TV broadcast, radio broadcast, and the like. Such communication systems are generally highly specialized purpose-build communication systems. With the recent, advancements in wireless cellular telephone systems there has been an interest of utilizing the existing infrastructure of the—mainly point-to-point cellular telephone systems for broadcast services. (As used herein, the term “cellular” systems encompasses communication systems utilizing both cellular and PCS frequencies.)
The information signal to be exchanged among the terminals in a communication system is often organized into a plurality of packets. For the purposes of this description, a packet is a group of bytes, including data (payload) and control elements, arranged into a specific format. The control elements comprise, e.g., a preamble and a quality metric. The quality metric comprises, e.g., cyclical redundancy check (CRC), parity bit(s), and other types of metric known to one skilled in the art. The packets are usually formatted into a message in accordance with a communication channel structure. The message, appropriately modulated, traveling between the origination terminal and the destination terminal, is affected by characteristics of the communication channel, e.g., signal-to-noise ratio, fading, time variance, and other such characteristics. Such characteristics affect the modulated signal differently in different communication channels. Consequently, transmission of a modulated signal over a wireless communication channel requires different considerations than transmission of a modulated signal over a wire-like communication channel, e.g., a coaxial cable or an optical cable.
In addition to selecting a modulation appropriate for a particular communication channel, other methods for protecting the information signal have been devised. Such methods comprise, e.g., encoding, symbol repetition, interleaving, and other methods know to one of ordinary skill in the art. However, these methods increase overhead. Therefore, an engineering compromise between reliability of message delivery and the amount of overhead must be made. Even with the above-discussed protection of information, the conditions of the communication channel can degrade to the point at which the destination station possibly cannot decode (erases) some of the packets comprising the message. In data-only communications systems, the cure is to re-transmit the non-decoded packets using an Automatic Retransmission reQuest (ARQ) made by the destination station to the origination station. However, as discussed, the subscribers do not communicate back to the base station. Furthermore, even if the subscribers were allowed to communicate ARQ, this communication might overload the communication system. Consequently, other means of information protection are desirable.
An outer code and an inner code are utilized in communication systems to provide information protection. An outer code comprises an outer encoder at a base station controller and an outer decoder at a subscriber station. An inner code comprises an inner encoder at a base station and an inner decoder at a subscriber station. The outer code and inner code involve encoding and decoding a block of information. The outer and inner code adds redundant information to improve protection. The redundancy permits decoding of the information from less than a complete encoded block of information.
A bit stream of information to be transmitted is provided to transmit buffers and encoded by an outer decoder communicatively coupled to the transmit buffers. Redundant bits are provided to each transmit buffer. Then, the content of the transmit buffers is multiplexed and encoded by an inner encoder to improve information protection further. The receiving subscriber station recovers the transmitted information by an inverse process, i.e., decoding.
Co-pending application Ser. No. 09/933,912, entitled “Method and System for Utilization of an Outer Decoder in a Broadcast Services Communication System,” filed Aug. 20, 2001, and assigned to the assignee of the present invention, discussed in detail utilization of an outer decoder in a broadcast system. Co-pending application Ser. No. 10/226/058, entitled “Method and System for COMMUNICATING CONTENT ON A BROADCAST SERVICES COMMUNICATION SYSTEM,” filed Aug. 21, 2002, and assigned to the assignee of the present invention, also discussed in detail utilization of an outer decoder in a broadcast system and focused on time re-alignment of two transmissions of the same content from two base stations to mitigate the problem of clipped frames.
Co-pending application Ser. No. 09/976,591, entitled “Method and System for REDUCTION OF DECODING COMPLEXITY IN A COMMUNICATION SYSTEM,” filed Oct. 12, 2001, and assigned to the assignee of the present invention, discussed in detail the transmit buffers of the outer code in a broadcast system.
Even with an outer code and inner code, a subscriber station may not be able to create the bit stream of information from decoded received packets from a particular base station. Thus, there is a need in the art for a method and a system that enables creating the bit stream of information even when a subscriber station cannot create the bit stream of information from decoded packets received from a particular base station.